Transversely-excited film bulk acoustic resonators with molybdenum conductors

ABSTRACT

There is disclosed acoustic resonators and filter devices. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The interleaved fingers of the IDT are substantially molybdenum. The piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm. A thickness of the interleaved fingers of the IDT is between 0.25 times and 2.5 times a thickness of the piezoelectric plate.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application62/926,181, filed Oct. 25, 2019, entitled WIDE BAND TRANSVERSELY-EXCITEDBULK ACOUSTIC WAVE RESONATORS WITH LOW LOSS ELECTRODES. This patent is acontinuation-in-part of application Ser. No. 16/578,811, filed Oct. 23,2019, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS FORHIGH POWER APPLICATIONS, which is a continuation-in-part of applicationSer. No. 16/230,443, filed Dec. 21, 2018, entitled TRANSVERSELY-EXCITEDFILM BULK ACOUSTIC RESONATOR, now U.S. Pat. No. 10,491,192, which claimspriority from the following provisional patent applications: application62/685,825, filed Jun. 15, 2018, entitled SHEAR-MODE FBAR (XBAR);application 62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODE FBAR(XBAR); application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZLATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883,filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR;and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUMTANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR. All of theseapplications are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to bandpass filters with high powercapability for use in communications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is less than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. The 5G NR standard also defines millimeter wave communicationbands with frequencies between 24.25 GHz and 40 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3A is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 3B is another alternative schematic cross-sectional view of theXBAR of FIG. 1.

FIG. 3C is an alternative schematic plan view of an XBAR

FIG. 4 is a graphic illustrating a primary acoustic mode in an XBAR.

FIG. 5 is a schematic circuit diagram of a band-pass filter usingacoustic resonators in a ladder circuit.

FIG. 6 is a graph showing the relationship between piezoelectricdiaphragm thickness and resonance frequency of an XBAR.

FIG. 7 is a plot showing the relationship between coupling factor Gamma(F) and IDT pitch for an XBAR.

FIG. 8 is a graph showing the dimensions of XBAR resonators withcapacitance equal to one picofarad.

FIG. 9 is a graph showing the relationship between IDT finger pitch andresonance and anti-resonance frequencies of an XBAR, with dielectriclayer thickness as a parameter.

FIG. 10 is a graph comparing the admittances of three simulated XBARswith different IDT metal thicknesses.

FIG. 11 is a graph illustrating the effect of IDT finger width onspurious resonances in an XBAR.

FIG. 12 is a graph comparing XBARs with aluminum and molybdenum IDTfingers.

FIG. 13 is an expanded portion of the graph of FIG. 12 comparing XBARswith aluminum and molybdenum IDT fingers.

FIG. 14 is a graph identifying preferred combinations of molybdenum IDTthickness and IDT pitch for XBARs without a front dielectric layer.

FIG. 15 is a graph identifying preferred combinations of molybdenum IDTthickness and IDT pitch for XBARs with front dielectric layer thicknessequal to 0.25 times the XBAR diaphragm thickness.

FIG. 16 is a cross-section view and two detailed cross-sectional viewsof a portion of an XBAR with two-layer IDT fingers.

FIG. 17 is a graph of S-parameters S11 and S21 of an exemplary bandpassfilter using XBARs with molybdenum conductors.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the front and back surfaces 112, 114.However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”. In other configurations, the diaphragm 115 may be contiguous withthe piezoelectric plate are at least 50% of the perimeter 145 of thecavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 under the diaphragm 115. The cavity 140 may be formed, for example,by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the portion 115 of thepiezoelectric plate that spans, or is suspended over, the cavity 140. Asshown in FIG. 1, the cavity 140 has a rectangular shape with an extentgreater than the aperture AP and length L of the IDT 130. A cavity of anXBAR may have a different shape, such as a regular or irregular polygon.The cavity of an XBAR may more or fewer than four sides, which may bestraight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds ofparallel fingers in the IDT 110. Similarly, the thickness of the fingersin the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. Thickness ts may be, for example, 100 nmto 1500 nm. When used in filters for LTE™ bands from 3.4 GHz to 6 GHz(e.g. bands 42, 43, 46), the thickness ts may be, for example, 200 nm to1000 nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 may be formed only between the IDT fingers (e.g. IDT finger238 b) or may be deposited as a blanket layer such that the dielectriclayer is formed both between and over the IDT fingers (e.g. IDT finger238 a). The front-side dielectric layer 214 may be a non-piezoelectricdielectric material, such as silicon dioxide or silicon nitride. tfd maybe, for example, 0 to 500 nm. tfd is typically less than the thicknessts of the piezoelectric plate. The front-side dielectric layer 214 maybe formed of multiple layers of two or more materials.

The IDT fingers 238 a and 238 b may be aluminum, an aluminum alloy,copper, a copper alloy, beryllium, gold, tungsten, molybdenum or someother conductive material. The IDT fingers are considered to be“substantially aluminum” if they are formed from aluminum or an alloycomprising at least 50% aluminum. The IDT fingers are considered to be“substantially copper” if they are formed from copper or an alloycomprising at least 50% copper. The IDT fingers are considered to be“substantially molybdenum” if they are formed from molybdenum or analloy comprising at least 50% molybdenum. Thin (relative to the totalthickness of the conductors) layers of other metals, such as chromium ortitanium, may be formed under and/or over and/or as layers within thefingers to improve adhesion between the fingers and the piezoelectricplate 110 and/or to passivate or encapsulate the fingers and/or toimprove power handling. The busbars (132, 134 in FIG. 1) of the IDT maybe made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The geometry of the IDT of an XBAR differs substantially fromthe IDTs used in surface acoustic wave (SAW) resonators. In a SAWresonator, the pitch of the IDT is one-half of the acoustic wavelengthat the resonance frequency. Additionally, the mark-to-pitch ratio of aSAW resonator IDT is typically close to 0.5 (i.e. the mark or fingerwidth is about one-fourth of the acoustic wavelength at resonance). Inan XBAR, the pitch p of the IDT is typically 2 to 20 times the width wof the fingers. In addition, the pitch p of the IDT is typically 2 to 20times the thickness is of the piezoelectric slab 212. The width of theIDT fingers in an XBAR is not constrained to be near one-fourth of theacoustic wavelength at resonance. For example, the width of XBAR IDTfingers may be 500 nm or greater, such that the IDT can be readilyfabricated using optical lithography. The thickness tm of the IDTfingers may be from 100 nm to about equal to the width w. The thicknessof the busbars (132, 134 in FIG. 1) of the IDT may be the same as, orgreater than, the thickness tm of the IDT fingers.

FIG. 3A and FIG. 3B show two alternative cross-sectional views along thesection plane A-A defined in FIG. 1. In FIG. 3A, a piezoelectric plate310 is attached to a substrate 320. A portion of the piezoelectric plate310 forms a diaphragm 315 spanning a cavity 340 in the substrate. Thecavity 340 does not fully penetrate the substrate 320. Fingers of an IDTare disposed on the diaphragm 315. The cavity 340 may be formed, forexample, by etching the substrate 320 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings (not shown) provided in the piezoelectricplate 310. In this case, the diaphragm 315 may contiguous with the restof the piezoelectric plate 310 around a large portion of a perimeter 345of the cavity 340. For example, the diaphragm 315 may contiguous withthe rest of the piezoelectric plate 310 around at least 50% of theperimeter 345 of the cavity 340. An intermediate layer (not shown), suchas a dielectric bonding layer, may be present between the piezo electricplate 340 and the substrate 320.

In FIG. 3B, the substrate 320 includes a base 322 and an intermediatelayer 324 disposed between the piezoelectric plate 310 and the base 322.For example, the base 322 may be silicon and the intermediate layer 324may be silicon dioxide or silicon nitride or some other material. Aportion of the piezoelectric plate 310 forms a diaphragm 315 spanning acavity 340 in the intermediate layer 324. Fingers of an IDT are disposedon the diaphragm 315. The cavity 340 may be formed, for example, byetching the intermediate layer 324 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching theintermediate layer 324 with a selective etchant that reaches thesubstrate through one or more openings provided in the piezoelectricplate 310. In this case, the diaphragm 315 may be contiguous with therest of the piezoelectric plate 310 around a large portion of aperimeter 345 of the cavity 340 (see FIG. 3C). For example, thediaphragm 315 may be contiguous with the rest of the piezoelectric plate310 around at least 50% of the perimeter 345 of the cavity 340 as shownin FIG. 3C. Although not shown in FIG. 3B, a cavity formed in theintermediate layer 324 may extend into the base 322.

FIG. 3C is a schematic plan view of another XBAR 350. The XBAR 350includes an IDT formed on a piezoelectric plate 310. A portion of thepiezoelectric plate 310 forms a diaphragm spanning a cavity in asubstrate. In this example, the perimeter 345 of the cavity has anirregular polygon shape such that none of the edges of the cavity areparallel, nor are they parallel to the conductors of the IDT. A cavitymay have a different shape with straight or curved edges.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430 which alternate in electrical polarity from finger to finger. An RFvoltage is applied to the interleaved fingers 430. This voltage createsa time-varying electric field between the fingers. The direction of theelectric field is predominantly lateral, or parallel to the surface ofthe piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the RF electric energy is highly concentratedinside the plate relative to the air. The lateral electric fieldintroduces shear deformation which couples strongly to a shear primaryacoustic mode (at a resonance frequency defined by the acoustic cavityformed by the volume between the two surfaces of the piezoelectricplate) in the piezoelectric plate 410. In this context, “sheardeformation” is defined as deformation in which parallel planes in amaterial remain predominantly parallel and maintain constant separationwhile translating (within their respective planes) relative to eachother. A “shear acoustic mode” is defined as an acoustic vibration modein a medium that results in shear deformation of the medium. The sheardeformations in the XBAR 400 are represented by the curves 460, with theadjacent small arrows providing a schematic indication of the directionand relative magnitude of atomic motion at the resonance frequency. Thedegree of atomic motion, as well as the thickness of the piezoelectricplate 410, have been greatly exaggerated for ease of visualization.While the atomic motions are predominantly lateral (i.e. horizontal asshown in FIG. 4), the direction of acoustic energy flow of the excitedprimary acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

Considering FIG. 4, there is essentially no RF electric fieldimmediately under the IDT fingers 430, and thus acoustic modes are onlyminimally excited in the regions 470 under the fingers. There may beevanescent acoustic motions in these regions. Since acoustic vibrationsare not excited under the IDT fingers 430, the acoustic energy coupledto the IDT fingers 430 is low (for example compared to the fingers of anIDT in a SAW resonator) for the primary acoustic mode, which minimizesviscous losses in the IDT fingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. Such devices are usually based on AlN thinfilms with the C axis of the AlN perpendicular to the surfaces of thefilm. The acoustic mode is compressive with atomic motions and thedirection of acoustic energy flow in the thickness direction. Inaddition, the piezoelectric coupling for shear wave XBAR resonances canbe high (>20%) compared to other acoustic resonators. High piezoelectriccoupling enables the design and implementation of microwave andmillimeter-wave filters with appreciable bandwidth.

FIG. 5 is a schematic circuit diagram of a band-pass filter 500 usingfive XBARs X1-X5. The filter 500 may be, for example, a band n79band-pass filter for use in a communication device. The filter 500 has aconventional ladder filter architecture including three seriesresonators X1, X3, X5 and two shunt resonators X2, X4. The three seriesresonators X1, X3, X5 are connected in series between a first port and asecond port. In FIG. 5, the first and second ports are labeled “In” and“Out”, respectively. However, the filter 500 is bidirectional and eitherport may serve as the input or output of the filter. The two shuntresonators X2, X4 are connected from nodes between the series resonatorsto ground. All the shunt resonators and series resonators are XBARs.

The three series resonators X1, X3, X5 and the two shunt resonators X2,X4 of the filter 500 maybe formed on a single plate 530 of piezoelectricmaterial bonded to a silicon substrate (not visible). Each resonatorincludes a respective IDT (not shown), with at least the fingers of theIDT disposed over a cavity in the substrate. In this and similarcontexts, the term “respective” means “relating things each to each”,which is to say with a one-to-one correspondence. In FIG. 5, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 535). In this example, an IDT of each resonator isdisposed over a respective cavity. In other filters, the IDTs of two ormore resonators may be disposed over a common cavity. Resonators mayalso be cascaded into multiple IDTs which may be formed on multiplecavities.

Each of the resonators X1 to X5 has a resonance frequency and ananti-resonance frequency. In simplified terms, each resonator iseffectively a short circuit at its resonance frequency and effectivelyan open circuit at its anti-resonance frequency. Each resonator X1 to X5creates a “transmission zero”, where the transmission between the in andout ports of the filter is very low. Note that the transmission at a“transmission zero” is not actually zero due to energy leakage throughparasitic components and other effects. The three series resonators X1,X3, X5 create transmission zeros at their respective anti-resonancefrequencies (where each resonator is effectively an open circuit). Thetwo shunt resonators X2, X4 create transmission zeros at theirrespective resonance frequencies (where each resonator is effectively ashort circuit). In a typical band-pass filter using acoustic resonators,the anti-resonance frequencies of the series resonators createtransmission zeros above the passband, and the resonance frequencies ofthe shunt resonators create transmission zeros below the passband.

A band-pass filter for use in a communications device, such as acellular telephone, must meet a variety of requirements. First, aband-pass filter, by definition, must pass, or transmit with acceptableloss, a defined pass-band. Typically, a band-pass filter for use in acommunications device must also stop, or substantially attenuate, one ormore stop band(s). For example, a band n79 band-pass filter is typicallyrequired to pass the n79 frequency band from 4400 MHz to 5000 MHz and tostop the 5 GHz WiFi™ band and/or the n77 band from 3300 MHz to 4200 MHz.To meet these requirements, a filter using a ladder circuit wouldrequire series resonators with anti-resonance frequencies about or above5100 MHz, and shunt resonators with resonance frequencies about or below4300 MHz.

The resonance and anti-resonance frequencies of an XBAR are stronglydependent on the thickness ts of the piezoelectric diaphragm (115 inFIG. 1). FIG. 6 is a graph 600 of resonance frequency of an XBAR versuspiezoelectric diaphragm thickness. In this example, the piezoelectricdiaphragm is z-cut lithium niobate. The solid curve 610 is plot ofresonance frequency as a function of the inverse of the piezoelectricplate thickness for XBARs with IDT pitch equal to 3 microns. This plotis based on results of simulations of XBARs using finite elementmethods. The resonance frequency is roughly proportional to the inverseof the piezoelectric plate thickness.

The resonance and anti-resonance frequencies of an XBAR are alsodependent on the pitch (dimension p in FIG. 2) of the IDT. Further, theelectromechanical coupling of an XBAR, which determines the separationbetween the resonance and anti-resonance frequencies, is dependent onthe pitch. FIG. 7 is a graph 700 of gamma (F) as a function ofnormalized pitch, which is to say IDT pitch p divided by diaphragmthickness ts. Gamma is a metric defined by the equation:

$\Gamma = \frac{1}{\left( {{Fa}\text{/}{Fr}} \right)^{2} - 1}$

where Fa is the antiresonance frequency and Fr is the resonancefrequency. Large values for gamma correspond to smaller separationbetween the resonance and anti-resonance frequencies. Low values ofgamma indicate strong coupling which is good for design of widebandladder filters.

In this example, the piezoelectric diaphragm is z-cut lithium niobate,and data is presented for diaphragm thicknesses of 300 nm, 400 nm, and500 nm. In all cases, the IDT is aluminum with a thickness of 25% of thediaphragm thickness, the duty factor (i.e. the ratio of the width w tothe pitch p) of the IDT fingers is 0.14, and there are no dielectriclayers. The “+” symbols, circles, and “x” symbols represent diaphragmthicknesses of 300 nm, 400 nm, and 500 nm, respectively. Outlier datapoints, such as those for relative IDT pitch about 4.5 and about 8, arecaused by spurious modes interacting with the primary acoustic mode andaltering the apparent gamma. The relationship between gamma and IDTpitch is relatively independent of diaphragm thickness, and roughlyasymptotic to 1=3.5 as the relative pitch is increased.

Another typical requirement on a band-pass filter for use in acommunications device is the input and output impedances of the filterhave to match, at least over the pass-band of the filter, the impedancesof other elements of the communications device to which the filter isconnected (e.g. a transmitter, receiver, and/or antenna) for maximumpower transfer. Commonly, the input and output impedances of a band-passfilter are required to match a 50-ohm impedance within a tolerance thatmay be expressed, for example, as a maximum return loss or a maximumvoltage standing wave ratio. When necessary, an impedance matchingnetwork comprising one or more reactive components can be used at theinput and/or output of a band-pass filter. Such impedance matchingnetworks add to the complexity, cost, and insertion loss of the filterand are thus undesirable. To match, without additional impedancematching components, a 50-Ohm impedance at a frequency of 5 GHz, thecapacitances of at least the shunt resonators in the band-pass filterneed to be in a range of about 0.5 picofarads (pF) to about 1.5picofarads.

FIG. 8 is a graph 800 showing the area and dimensions of XBAR resonatorswith capacitance equal to one picofarad. The solid line 810 is a plot ofthe IDT length required to provide a capacitance of 1 pF as a functionof the inverse of the IDT aperture when the IDT pitch is 3 microns. Thedashed line 820 is a plot of the IDT length required to provide acapacitance of 1 pF as a function of the inverse of the IDT aperturewhen the IDT pitch is 5 microns. The data plotted in FIG. 8 is specificto XBAR devices with lithium niobate diaphragm thickness of 400 nm.

For any aperture, the IDT length required to provide a desiredcapacitance is greater for an IDT pitch of 5 microns than for an IDTpitch of 3 microns. The required IDT length is roughly proportional tothe change in IDT pitch. The design of a filter using XBARs is acompromise between somewhat conflicting objectives. As shown in FIG. 7,a larger IDT pitch may be preferred to reduce gamma and maximize theseparation between the anti-resonance and resonance frequencies. As canbe understood from FIG. 8, smaller IDT pitch is preferred to minimizeIDT area. A reasonable compromise between these objectives is6≤p/ts≤12.5. Setting the IDT pitch p equal to or greater than six timesthe diaphragm thickness ts provides Fa/Fr greater than 1.1. Setting themaximum IDT pitch p to 12.5 times the diaphragm thickness ts isreasonable since Fa/Fr does not increase appreciably for higher valuesof relative pitch.

As will be discussed is greater detail subsequently, the metal fingersof the IDTs provide the primary mechanism for removing heat from an XBARresonator. Increasing the aperture of a resonator increases the lengthand the electrical and thermal resistance of each IDT finger. Further,for a given IDT capacitance, increasing the aperture reduces the numberof fingers required in the IDT, which, in turn, proportionally increasesthe RF current flowing in each finger. All of these effects argue forusing the smallest possible aperture in resonators for high-powerfilters.

Conversely, several factors argue for using a large aperture. First, thetotal area of an XBAR resonator includes the area of the IDT and thearea of the bus bars. The area of the bus bars is generally proportionalto the length of the IDT. For very small apertures, the area of the IDTbus bars may be larger than the area occupied by the interleaved IDTfingers. Further, some electrical and acoustic energy may be lost at theends of the IDT fingers. These loss effects become more significant asIDT aperture is reduced and the total number of fingers is increased.These losses may be evident as a reduction in resonator Q-factor,particularly at the anti-resonance frequency, as IDT aperture isreduced.

As a compromise between conflicting objectives, resonators apertureswill typically fall in the range from 20 μm and 60 μm for 5 GHzresonance frequency. Resonator aperture may scale inversely withfrequency.

The resonance and anti-resonance frequencies of an XBAR are alsodependent on the thickness (dimension tfd in FIG. 2) of the front-sidedielectric layer applied between (and optionally over) the fingers ofthe IDT. FIG. 9 is a graph 900 of anti-resonant frequency and resonantfrequency as a function of IDT finger pitch p for XBAR resonators withz-cut lithium niobate piezoelectric plate thickness is =400 nm, withfront-side SiO₂ dielectric layer thickness tfd as a parameter. The solidlines 910 and 920 are plots of the anti-resonance and resonancefrequencies, respectively, as functions of IDT pitch for tfd=0. Thedashed lines 912 and 922 are plots of the anti-resonance and resonancefrequencies, respectively, as functions of IDT pitch for tfd=30 nm. Thedash-dot lines 914 and 924 are plots of the anti-resonance and resonancefrequencies, respectively, as functions of IDT pitch for tfd=60 nm. Thedash-dot-dot lines 916 and 926 are plots of the anti-resonance andresonance frequencies, respectively, as functions of IDT pitch fortfd=90 nm. The frequency shifts are approximately linear functions oftfd.

In FIG. 9, the difference between the resonance and anti-resonancefrequencies is 600 to 650 MHz for any particular values for front-sidedielectric layer thickness and IDT pitch. This difference is largecompared to that of older acoustic filter technologies, such as surfaceacoustic wave filters. However, 650 MHz is not sufficient for very wideband filters such as band-pass filters needed for bands n77 and n79. Asdescribed in U.S. patent application Ser. No. 16/230,443, the front-sidedielectric layer over shunt resonators may be thicker than thefront-side dielectric layer over series resonators. A thicker dielectriclayer over shunt resonators shifts the resonance frequency of the shuntresonators downward and thus increases the frequency difference betweenthe resonant frequencies of the shunt resonators and the anti-resonancefrequencies of the series resonators.

Communications devices operating in time-domain duplex (TDD) bandstransmit and receive in the same frequency band. Both the transmit andreceive signal paths pass through a common bandpass filter connectedbetween an antenna and a transceiver. Communications devices operatingin frequency-domain duplex (FDD) bands transmit and receive in differentfrequency bands. The transmit and receive signal paths pass throughseparate transmit and receive bandpass filters connected between anantenna and the transceiver. Filters for use in TDD bands or filters foruse as transmit filters in FDD bands can be subjected to radio frequencyinput power levels of 30 dBm or greater and must avoid damage underpower.

The required insertion loss of acoustic wave bandpass filters is usuallynot more than a few dB. Some portion of this lost power is return lossreflected back to the power source; the rest of the lost power isdissipated in the filter. Typical band-pass filters for LTE bands havesurface areas of 1.0 to 2.0 square millimeters. Although the total powerdissipation in a filter may be small, the power density can be highgiven the small surface area. Further, the primary loss mechanisms in anacoustic filter are resistive losses in the conductor patterns andacoustic losses in the IDT fingers and piezoelectric material. Thus, thepower dissipation in an acoustic filter is concentrated in the acousticresonators. To prevent excessive temperature increase in the acousticresonators, the heat due to the power dissipation must be conducted awayfrom the resonators through the filter package to the environmentexternal to the filter.

In traditional acoustic filters, such as surface acoustic wave (SAW)filters and bulk acoustic wave (BAW) filters, the heat generated bypower dissipation in the acoustic resonators is efficiently conductedthrough the filter substrate and the metal electrode patterns to thepackage. In an XBAR device, the resonators are disposed on thinpiezoelectric diaphragms that are inefficient heat conductors. The largemajority of the heat generated in an XBAR device must be removed fromthe resonator via the IDT fingers and associated conductor patterns.

To minimize power dissipation and maximize heat removal, the IDT fingersand associated conductors should be formed from a material that has lowelectrical resistivity and high thermal conductivity. Some metals havingboth low resistivity and high thermal conductivity are listed in thefollowing table:

Electrical Thermal resistivity conductivity Metal (10⁻⁶ Ω-cm) (W/m-K)Silver 1.55 419 Copper 1.70 385 Gold 2.2 301 Aluminum 2.7 210 Molybdenum5.34 138

Silver offers the lowest resistivity and highest thermal conductivitybut is not a viable candidate for IDT conductors due to the lack ofprocesses for deposition and patterning of silver thin films.Appropriate processes are available for copper, gold, aluminum, andmolybdenum. Aluminum offers the most mature processes for use inacoustic resonator devices and potentially the lowest cost, but withhigher resistivity and reduced thermal conductivity compared to copperand gold. For comparison, the thermal conductivity of lithium niobate isabout 4 W/m-K, or about 2% of the thermal conductivity of aluminum.Aluminum also has good acoustic attenuation properties which helpsminimize dissipation.

Molybdenum has very low acoustic attenuation compared to the othermetals. The acoustic losses in molybdenum may be as little as 2% of theacoustic losses in equivalent aluminum electrodes. Molybdenum has thehighest electrical resistivity and lowest thermal conductivity of themetals listed in the table above.

The electric resistance of the IDT fingers can be reduced, and thethermal conductivity of the IDT fingers can be increased, by increasingthe cross-sectional area of the fingers to the extent possible. Asdescribed in conjunction with FIG. 4, unlike SAW or BAW, for XBAR thereis little coupling of the primary acoustic mode to the IDT fingers.Changing the width and/or thickness of the IDT fingers has minimaleffect on the primary acoustic mode in an XBAR device. This is a veryuncommon situation for an acoustic wave resonator. However, the IDTfinger geometry does have a substantial effect on coupling to spuriousacoustic modes, such as higher order shear modes and plate modes thattravel laterally in the piezoelectric diaphragm.

FIG. 10 is a chart 1000 illustrating the effect that IDT fingerthickness can have on XBAR performance. The solid curve 1010 is a plotof the magnitude of the admittance of an XBAR device with the thicknessof the IDT fingers tm=100 nm. The dashed curve 1030 is a plot of themagnitude of the admittance of an XBAR device with the thickness of theIDT fingers tm=250 nm. The dot-dash curve 1020 is a plot of themagnitude of the admittance of an XBAR device with the thickness of theIDT fingers tm=500 nm. The three curves 1010, 1020, 1030 have beenoffset vertically by about 1.5 units for visibility. The three XBARdevices are identical except for the thickness of the IDT fingers. Thepiezoelectric plate is lithium niobate 400 nm thick, the IDT electrodesare aluminum, and the IDT pitch is 4 microns. The XBAR devices withtm=100 nm and tm=500 nm have similar resonance frequencies, Q-factors,and electromechanical coupling. The XBAR device with tm=250 nm exhibitsa spurious mode at a frequency near the resonance frequency, such thatthe resonance is effectively split into two low Q-factor, low admittancepeaks separated by several hundred MHz. The XBAR with tm=250 nm (curve1030) is not be useable in a filter.

FIG. 11 is a chart 1100 illustrating the effect that IDT finger width wcan have on XBAR performance. The solid curve 1110 is a plot of themagnitude of the admittance of an XBAR device with the width of the IDTfingers w=0.74 micron. Note the spurious mode resonance at a frequencyabout 4.9 GHz, which could lie within the pass-band of a filterincorporating this resonator. Such effects could cause an unacceptableperturbation in the transmittance within the filter passband. The dashedcurve 1120 is a plot of the magnitude of the admittance of an XBARdevice with the width of the IDT fingers w=0.86 micron. The tworesonators are identical except for the dimension w. The piezoelectricplate is lithium niobate 400 nm thick, the IDT electrodes are aluminum,and the IDT pitch is 3.25 microns. Changing w from 0.74 micron to 0.86micron suppressed the spurious mode with little or no effect onresonance frequency and electromechanical coupling.

As previously described, molybdenum IDT fingers will have much loweracoustic losses than aluminum or copper IDT fingers. Although theelectrical and thermal conductivity of molybdenum is low compared toother metals, molybdenum IDT electrodes can be as thick as required toachieve adequate thermal and electrical conductivity. FIG. 12 is a chart1200 comparing the magnitude of the admittance of XBARs with molybdenumand aluminum IDT fingers. The solid line 1210 is a plot of the magnitudeof admittance of a representative XBAR with molybdenum IDT fingers as afunction of frequency. The dashed line 1220 is a plot of the magnitudeof admittance of a representative XBAR with aluminum IDT fingers as afunction of frequency. In both cases, the thickness of the IDT fingersis sufficient to reduce resistive losses to a negligible level, and thewidth of the IDT fingers is selected to minimize spurious effect betweenthe resonance and anti-resonance frequencies. Comparison of the twoplots shows that the XBAR with molybdenum IDT fingers (solid curve 1210)has sharper resonance and anti-resonance peaks than the XBAR withaluminum fingers (dashed curve 1220). Sharper resonance andanti-resonance peaks are indicative of significantly higher Q-factor.The higher Q-factor is primarily due to the lower acoustic losses in themolybdenum. The XBAR with molybdenum fingers (solid curve 1210) also hasmore and larger spurious effects, which may imply that acoustic lossesdamp or attenuate spurious effects in the XBAR with aluminum fingers(dashed curve 1220).

FIG. 13 is a chart 1300 showing an expanded portion of the chart 1200 ofFIG. 12. The solid line 1310 is a plot of the magnitude of admittance ofan XBAR with molybdenum IDT fingers as a function of frequency. Thedashed line 1320 is a plot of the magnitude of admittance of an XBARwith aluminum IDT fingers as a function of frequency. Both curves showthe admittance peak that occurs at the resonance frequency of the XBARs.Comparison of the two plots shows that the XBAR with molybdenum IDTfingers (solid curve 1310) has a sharper and higher admittance peak,which is indicative of significantly higher Q-factor, than the XBAR withaluminum fingers (dashed curve 1320). The higher Q-factor is primarilydue to the lower acoustic losses in the molybdenum.

Given the complex dependence of spurious mode frequency and amplitude ondiaphragm thickness ts, IDT metal thickness tm, IDT pitch p and IDTfinger width w, the inventors undertook an empirical evaluation, usingtwo-dimensional finite element modeling, of a large number ofhypothetical XBAR resonators. For each combination of diaphragmthickness ts, IDT finger thickness tm, and IDT pitch p, the XBARresonator was simulated for a sequence of IDT finger width w values. Afigure of merit (FOM) was calculated for each value of w to estimate thenegative impact of spurious modes. The FOM is calculated by integratingthe negative impact of spurious modes across a defined frequency range.The FOM and the frequency range depend on the requirements of aparticular filter. The frequency range typically includes the passbandof the filter and may include one or more stop bands. Spurious modesoccurring between the resonance and anti-resonance frequencies of eachhypothetical resonator were given a heavier weight in the FOM thanspurious modes at frequencies below resonance or above anti-resonance.Hypothetical resonators having a minimized FOM below a threshold valuewere considered potentially “useable”, which is to say probably havingsufficiently low spurious modes for use in a filter. Hypotheticalresonators having a minimized FOM above the threshold value wereconsidered not useable.

U.S. patent application Ser. No. 16/578,811, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS FOR HIGH POWERAPPLICATIONS, contains charts showing combinations of IDT pitch and IDTfinger thickness that may provide useable resonators with aluminum andcopper IDT conductors. FIG. 14 and FIG. 15 of the present patent aresimilar charts for molybdenum conductors.

FIG. 14 is a chart 1400 showing combinations of IDT pitch and IDT fingerthickness that may provide useable resonators. This chart is based ontwo-dimensional simulations of XBARs with lithium niobate diaphragmthickness is =400 nm, molybdenum conductors, and front-side dielectricthickness tfd=0. XBARs with IDT pitch and thickness within shadedregions 1410, 1420, 1430 are likely to have sufficiently low spuriouseffects for use in filters. For each combination of IDT pitch and IDTfinger thickness, the width of the IDT fingers was selected to minimizethe FOM. The black dots 1450 represent XBARs used in a filter to bediscussed subsequently. Resonators with acceptably low spurious effectsexist for IDT finger thickness greater than or equal to 0.25 times thediaphragm thickness and less than or equal to 2.5 times the diaphragmthickness. However, low spurious effects are not the only requirement.Resonators for use in filters must also have acceptably low resistivelosses and adequate removal of heat from the IDT and diaphragm.Resonators for use in filters will typically have IDT finger thicknessgreater than or equal to the diaphragm thickness and less than or equalto 2.5 times the diaphragm thickness.

As previously discussed, wide bandwidth filters using XBARs may use athicker front-side dielectric layer on shunt resonators than on seriesresonators to lower the resonance frequencies of the shunt resonatorswith respect to the resonance frequencies of the series resonators. Thefront-side dielectric layer on shunt resonators may be as much asseveral hundred nm thicker than the front side dielectric on seriesresonators. For ease of manufacturing, it may be preferable that thesame IDT finger thickness be used on both shunt and series resonators.

FIG. 15 is another chart 1500 showing combinations of IDT pitch and IDTfinger thickness that may provide useable resonators. This chart isbased on simulations of XBARs with lithium niobate diaphragmthickness=400 nm, molybdenum conductors, and tfd=100 nm (0.25 times thediaphragm thickness). XBARs having IDT pitch and thickness within shadedregions 1510, 1520, 1530 are likely to have sufficiently low spuriouseffects for use in filters. For each combination of IDT pitch and IDTfinger thickness, the width of the IDT fingers was selected to minimizethe FOM. The black dots 1550 represent XBARs used in a filter to bediscussed subsequently. Usable resonators exist for IDT finger thicknessgreater than or equal to 0.25 times the diaphragm thickness and lessthan or equal to 2.5 times the diaphragm thickness, except that usableresonators do not exist for IDT thicknesses between about 0.4 times thediaphragm thickness to 0.65 times the diaphragm thickness.

Assuming that a filter is designed with no front-side dielectric layeron series resonators and 100 nm of front-side dielectric on shuntresonators, FIG. 14 and FIG. 15 jointly define the combinations of metalthickness and IDT pitch that result in useable resonators. Specifically,FIG. 14 defines useable combinations of metal thickness and IDT pitchfor series resonators, and FIG. 15 defines useable combinations of metalthickness and IDT for shunt resonators. Since only a single metalthickness is desirable for ease of manufacturing, the overlap betweenthe ranges defined in FIG. 14 and FIG. 15 defines the range of metalthicknesses for filter using a front-side dielectric to shift theresonance frequency of shunt resonator. Comparing FIG. 14 and FIG. 15,IDT molybdenum thickness between 0.65 times the diaphragm thickness and2.5 times the diaphragm thickness provides at least two differentuseable pitch values for both series and shunt resonators. A largerminimum finger thickness may be dictated by consideration of resistivelosses and heat removal. Resonators for use in filters will typicallyhave IDT finger thickness greater than or equal to the diaphragmthickness and less than or equal to 2.5 times the diaphragm thickness.

FIG. 16 is a cross-sectional view of a portion of an XBAR with two-layerIDT fingers. FIG. 16 shows a cross section though a portion of apiezoelectric diaphragm 1610 and two IDT finger 1620. Each IDT finger1620 has two metal layers 1630, 1640. The lower (as shown in FIG. 16)layer 1630 may be molybdenum or another metal with low acoustic loss,such as tungsten. The lower layer 1630 may be adjacent the diaphragm1610 or separated from the diaphragm 1610 by a thin intermediate layer(not shown) used to improve adhesion between the diaphragm 1610 and thelower layer 1630. The upper layer 1640 may be a material such asaluminum, copper, or gold having high electrical and thermalconductivity. The use of a metal with low acoustic losses for the lowerlayer 1630 closest to the piezoelectric diaphragm 1610, where theacoustic stresses are greatest, reduces acoustic losses in the XBAR. Theaddition of an upper layer 1640 of high conductivity metal can reduceelectrical losses and improve thermal conductivity. Having two metallayers 1630, 1640 allows the designer to have somewhat independentcontrol of acoustic and electrical losses in the XBAR.

Further, the two metal layers need not have the same thickness orcross-sectional shape, as shown in Detail A and Detail B of FIG. 16. InDetail A, the second metal layer 1640A of each IDT finger has the formof a narrow rib on top of a thinner, wider first metal layer 1630A. InDetail B, each IDT finger has a “T” cross section form by a narrow firstmetal layer 1630B and a wider second metal layer 1640B. Thecross-section shapes of the first and second metal layers are notlimited to rectangular as shown in FIG. 16. Other cross-sectional shapesincluding trapezoidal and (at least for the second metal layer)triangular may be used and may be beneficial to minimize or controlspurious acoustic modes.

FIG. 17 is a chart 1700 showing simulated performance of an exemplaryhigh-power XBAR band-pass filter for band n79. The circuit of theband-pass filter is a five-resonator ladder filter, similar to that ofFIG. 5. The XBARs are formed on a Z-cut lithium niobate plate. Thethickness is of the lithium niobate plate is 400 nm. The substrate issilicon, the IDT conductors are molybdenum, the front-side dielectric,where present, is SiO2. The thickness tm of the IDT fingers is 500 nm,such that tm/ts=1.25. The other physical parameters of the resonatorsare provided in the following table (all dimensions are in microns;p=IDT pitch, w=IDT finger width, AP=aperture, L=length, andtfd=front-side dielectric thickness):

Series Resonators Shunt Resonators Parameter X1 X3 X5 X2 X4 p 4.02 4.23.1 4.475 4.275 w 0.875 0.95 .925 .925 .675 AP 41.2 55.4 19.4 44.5 45.8L 520 250 235 350 325 tfd 0 0 0 0.100 0.100The series resonators correspond to the filled circles 1350 in FIG. 13,and the shunt resonators correspond to the filled circles 1450 in FIG.14.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substratehaving a surface; a single-crystal piezoelectric plate having front andback surfaces, the back surface attached to the surface of the substrateexcept for a portion of the piezoelectric plate forming a diaphragm thatspans a cavity in the substrate; and an interdigital transducer (IDT)formed on the front surface of the single-crystal piezoelectric platesuch that interleaved fingers of the IDT are disposed on the diaphragm,the piezoelectric plate and the IDT configured such that a radiofrequency signal applied to the IDT excites a primary shear acousticmode in the diaphragm, wherein the interleaved fingers of the IDT aresubstantially molybdenum, and a thickness of the interleaved fingers ofthe IDT is greater than or equal to 0.25 times a thickness of thepiezoelectric plate and less than or equal to 2.5 times the thickness ofthe piezoelectric plate.
 2. The acoustic resonator device of claim 1,wherein the thickness of the interleaved fingers of the IDT is greaterthan or equal to the thickness of the piezoelectric plate and less thanor equal to 2.5 times the thickness of the piezoelectric plate.
 3. Theacoustic resonator device of claim 1, further comprising: a front-sidedielectric layer deposited between the fingers of the IDT, wherein thethickness of the interleaved fingers of the IDT is greater than or equalto 0.65 times the thickness of the piezoelectric plate and less than orequal to 2.5 times the thickness of the piezoelectric plate.
 4. Theacoustic resonator device of claim 3, wherein the thickness of theinterleaved fingers of the IDT is greater than or equal to the thicknessof the piezoelectric plate and less than or equal to 2.5 times thethickness of the piezoelectric plate.
 5. The acoustic resonator deviceof claim 1, wherein the thickness of the piezoelectric plate is greaterthan or equal to 300 nm and less than or equal to 500 nm.
 6. Theacoustic resonator device of claim 1, wherein a pitch of the interleavedfingers of the IDT is greater than or equal to 6 times the thickness ofthe piezoelectric plate and less than or equal to 12.5 times thethickness of the piezoelectric plate.
 7. The acoustic resonator deviceof claim 1, wherein a direction of acoustic energy flow of the primaryacoustic mode is substantially normal to the front and back surfaces ofthe diaphragm.
 8. The acoustic resonator device of claim 1, wherein thediaphragm is contiguous with the piezoelectric plate around at least 50%of a perimeter of the cavity.
 9. An acoustic resonator devicecomprising: a substrate having a surface; a single-crystal piezoelectricplate having front and back surfaces, the back surface attached to thesurface of the substrate except for a portion of the piezoelectric plateforming a diaphragm that spans a cavity in the substrate; and aninterdigital transducer (IDT) formed on the front surface of thesingle-crystal piezoelectric plate such that interleaved fingers of theIDT are disposed on the diaphragm, the piezoelectric plate and the IDTconfigured such that a radio frequency signal applied to the IDT excitesa primary shear acoustic mode in the diaphragm, wherein the interleavedfingers of the IDT comprise a lower layer closest to the diaphragm of afirst material and an upper layer of a second material, the firstmaterial having lower acoustic loss than the second material and thesecond material having higher electrical and thermal conductivity thanthe first material.
 10. The acoustic resonator device of claim 9,wherein the first material is one of molybdenum and tungsten, and thesecond material is one of aluminum, copper, and gold.
 11. The acousticresonator device of claim 9, wherein a thickness of the lower layer isdifferent from a thickness of the upper layer.
 12. The acousticresonator device of claim 9, wherein a cross-sectional shape of thelower layer is different from a cross-sectional shape of the upperlayer.
 13. A filter device, comprising: a substrate; a single-crystalpiezoelectric plate having front and back surfaces, the back surfaceattached to the surface of the substrate, portions of the single-crystalpiezoelectric plate forming one or more diaphragms spanning respectivecavities in the substrate; and a conductor pattern formed on the frontsurface, the conductor pattern including a plurality of interdigitaltransducers (IDTs) of a respective plurality of acoustic resonators,interleaved fingers of each of the plurality of IDTs disposed on one ofthe one or more diaphragms, the piezoelectric plate and all of the IDTsconfigured such that respective radio frequency signal applied to eachIDT excite respective shear primary acoustic modes in the respectivediaphragm, wherein the interleaved fingers of all of the plurality ofIDTs are substantially molybdenum, and the interleaved fingers of all ofthe plurality of IDTs have a common finger thickness, which is greaterthan or equal to 0.25 times a thickness of the piezoelectric plate andless than or equal to 2.5 times the thickness of the piezoelectricplate.
 14. The filter device of claim 13, further comprising: afront-side dielectric layer deposited between the fingers of at leastsome, but not all, of the plurality of IDTs, wherein the common fingerthickness is greater than or equal to 0.65 times the thickness of thepiezoelectric plate and less than or equal to 2.5 times the thickness ofthe piezoelectric plate.
 15. The filter device of claim 14, wherein thecommon finger thickness is greater than or equal to the thickness of thepiezoelectric plate and less than or equal to 2.5 times the thickness ofthe piezoelectric plate.
 16. The filter device of claim 13, wherein thethickness of the piezoelectric plate is greater than or equal to 300 nmand less than or equal to 500 nm.
 17. The filter device of claim 13,wherein respective pitches of the interleaved fingers of all of theplurality of IDTs are greater than or equal to 6 times the thickness ofthe piezoelectric plate and less than or equal to 12.5 times thethickness of the piezoelectric plate.
 18. The filter device of claim 13,wherein a direction of acoustic energy flow of the respective primaryacoustic modes excited by all of the IDTs is substantially normal to thefront and back surfaces of the diaphragm.
 19. The filter device of claim13, wherein the diaphragms of all of the plurality of acoustic resonatorare contiguous with the piezoelectric plate around at least 50% of aperimeter of the respective cavities.
 20. A filter device, comprising: asubstrate; a single-crystal piezoelectric plate having front and backsurfaces, the back surface attached to the surface of the substrate,portions of the single-crystal piezoelectric plate forming one or morediaphragms spanning respective cavities in the substrate; a conductorpattern formed on the front surface, the conductor pattern including aplurality of interdigital transducers (IDTs) of a respective pluralityof acoustic resonators, interleaved fingers of each of the plurality ofIDTs disposed on one of the one or more diaphragms, the plurality ofresonators including one or more shunt resonators and one or more seriesresonators; a first dielectric layer having a first thickness depositedbetween the fingers of the IDTs of the one or more shunt resonators; anda second dielectric layer having a second thickness less than the firstthickness deposited between the fingers of the IDTs of the one or moreseries resonators, wherein the interleaved fingers of all of theplurality of IDTs are substantially molybdenum, and the interleavedfingers of all of the plurality of IDTs have a common thickness, whichis greater than or equal to 0.65 times the thickness of thepiezoelectric plate and less than or equal to 2.5 times the thickness ofthe piezoelectric plate.
 21. The filter device of claim 20, wherein thecommon thickness is greater than the thickness of the piezoelectricplate and less than 2.5 times the thickness of the piezoelectric plate.22. The filter device of claim 20, wherein the thickness of thepiezoelectric plate is greater than or equal to 300 nm and less than orequal to 500 nm.
 23. The filter device of claim 20, wherein respectivepitches of the interleaved fingers of all of the plurality of IDTs aregreater than or equal to 6 times the thickness of the piezoelectricplate and less than or equal to 12.5 times the thickness of thepiezoelectric plate.
 24. The filter device of claim 20, wherein adirection of acoustic energy flow of the respective primary acousticmodes excited by all of the plurality of IDTs is substantiallyorthogonal to the front and back surfaces of the diaphragm.
 25. Theacoustic resonator device of claim 20, wherein the diaphragm of each ofthe plurality of acoustic resonator is contiguous with the piezoelectricplate around at least 50% of a perimeter of the respective cavity.